Soft magnetic powder and magnetic shield composition

ABSTRACT

Soft magnetic powder comprising flat soft magnetic particles of an alloy having a composition defined and encompassed by polygon ABCDE in a Fe-Si-Cr ternary composition diagram of FIG. 1 or polygon JKLMN in a Fe-Si-Al ternary composition diagram of FIG. 4 is suitable for use in magnetic shields. The flat soft magnetic particles are prepared by furnishing alloy particles having a predetermined composition, flattening them, and heat treating the flat particles to develop a peak corresponding to plane index (002) in an X-ray diffraction diagram thereof.

This invention relates to soft magnetic powder for use in magneticshields, a method for preparing the same, and magnetic shieldcompositions containing the same.

BACKGROUND OF THE INVENTION

Magnetic shields are generally used for preventing influence of magneticfield-generating sources such as magnetized articles on other articlesor electric circuits. A commonly used class of magnetic shields aresheet metals having high magnetic permeability and hence, high shieldingproperties although the sheet metals have only limited versatility inview of nature and cost.

Another class of magnetic shields are powder materials which can beadvantageously applied in various ways. For example, magnetic powder isdispersed in organic binders to form coating compositions which areeither directly applicable to sites to be shielded against magnetism orcoated onto suitable flexible supports to form shielding plates.

A number of high magnetic permeability powders were proposed as magneticshield materials.

Japanese Patent Application Kokai No. 201493/1984, for example,discloses a magnetic shield coating composition comprising flatparticles obtained by finely dividing a soft magnetic amorphous alloyand a polymeric binder. Japanese Patent Application Kokai No. 59268/1983discloses a magnetic shield coating composition comprising flatparticles of a high magnetic permeability alloy dispersed in a polymericbinder. Japanese U.M. Publication No. 50495/1983 discloses to coatSendust alloy flakes to form a magnetic shield film. Japanese PatentPublication No. 58631/1987 discloses a magnetic shield coatingcomposition comprising flat, irregularly shaped particles dispersed in apolymeric binder, the particles being of Fe-Ni alloy, Fe-Ni-Co alloy,Fe-Si-Al alloy, and Fi-Ni-Mo alloy, which are commercially available asPermalloy, Molybdenum Permalloy, and Sendust alloy. Japanese PatentPublication No. 39966/1988 also discloses Permalloy magnetic shieldfilms. Further, Japanese Patent Application Kokai No. 223627/1989discloses magnetic shield protective films which are prepared by coatingflat magnetic iron powder consisting of iron and either one of 0.5 to20% by weight of Cr, 0.5 to 9% by weight (or 1 to 16.5 atom%) of Si, and0.5 to 15% by weight of Al.

Flat alloy particles are often used in these magnetic shield films andcompositions for the following reason. In coating such compositions,flat alloy particles are oriented such that their major surface isparallel to the coating surface. This means that the direction offlatness of particles coincides with the direction of magnetic shieldson use, allowing the magnetic shields to take full advantage of the highmagnetic permeability of the particles themselves due to the reduceddiamagnetic field attributable to the flat geometry. Good magneticshielding properties are provided since any loss of magnetic propertiesin a direction parallel to the coating surface by the influence ofdiamagnetic field is avoided.

Nevertheless, the conventional well-known alloy powders for magneticshields have several problems.

Among Fe-Si-Al alloys, one having the composition of 9.6 wt % Si, 5.4 wt% Al, and the balance of Fe and exhibiting a highest maximumpermeability μm is designated Sendust alloy. Sendust alloy suffers frominconvenience of handling because it is less corrosion resistant and inparticular, becomes pyrophoric when divided into flat shape because ofan increased specific surface area. It is also prone to rusting so thatit detracts from magnetic properties and outer appearance. In addition,Sendust alloy has a saturation magnetostriction constant which is lessthan about 0.3×10⁻⁶, but cannot be negative or lower than 0. When it isdesired to use Sendust alloy as magnetic shielding material byflattening it into, flat particles, its magnetic properties can bedeteriorated by stresses applied during flattening process and use,failing to meet the magnetic shield design requirement.

Permalloy type alloys including Permalloy and Molybdenum Permalloy areflattened through a rolling process rather than cleavage because oftheir crystal structure and thus require a longer time to flatten,leading to low productivity. The increased time of flattening processinduces more stresses in particles, failing to provide high magneticshielding properties. In addition, the Permalloy type alloys are about 5to 10 times more expensive than the Sendust alloy.

Iron base amorphous alloys also give rise to problems as found with thePermalloy type alloys since they are flattened through rolling.Moreover, Permalloy type alloys and iron base amorphous alloys haveincreased magnetostriction and thus detract from magnetic properties notonly through stress application during flattening, but also throughstress application during milling with binder to form a coatingcomposition. Another drawback of Permalloy type alloys is associatedwith their softness in that flat particles are liable to deform bystresses induced during milling to form a coating composition, alsoresulting in a loss of magnetic properties.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a novel andimproved soft magnetic powder for magnetic shields which has highcorrosion resistance and reduced magnetostriction. Another object of thepresent invention is to provide a novel and improved soft magneticpowder for magnetic shields which has reduced magnetostriction andstability against stresses. A further object is to provide a method forpreparing such soft magnetic powder by flattening a source material in arapid and efficient manner. A still further object is to provide a costeffective magnetic shield composition which contains such soft magneticpowder and exhibits enhanced magnetic shielding effect.

According to a first aspect of the present invention, there is provideda soft magnetic powder for use in magnetic shields. In one embodiment,the powder is in the form of flat soft magnetic particles of an alloyhaving a composition defined and encompassed by polygon ABCDE in aternary composition diagram of FIG. 1 comprised of Fe, Si, and Crwherein points A, B, C, D, and E have the following compositions asexpressed in atomic percentage.

A: Fe₇₈ Si₂₂ Cr₀

B: Fe₇₀ Si₃₀ Cr₀

C: Fe₆₀ Si₃₀ Cr₁₀

D: Fe₆₃ Si₁₈ Cr₁₉

E: Fe₇₆ Si₁₈ Cr₆

The powder in another embodiment is in the form of flat soft magneticparticles of an alloy having a composition defined and encompassed bypolygon JKLMN in a ternary composition diagram of FIG. 4 comprised ofFe, Si, and Al wherein points J, K, L, M, and N have the followingcompositions as expressed in atomic percentage.

J: Fe₈₃.8 Si₉.2 Al₇

K: Fe₈₄.7 Si₉.3 Al₆

L: Fe₈₅.6 Si₁₀.4 Al₄.0

M: Fe₈₄.9 Si₁₁.1 Al₄.0

N: Fe₈₃.2 Si₉.8 Al₇

In either of the embodiments, the flat soft magnetic particles show apeak corresponding to plane index (002) in an X-ray diffraction diagramthereof. More preferably, the flat soft magnetic particles meetP(002)/P(022)≧0.1% wherein P(002) is a peak height corresponding toplane index (002) and P(022) is a peak height corresponding to planeindex (022) in the X-ray diffraction diagram.

Preferably, the alloy has a saturation magnetostriction constant λs ofzero or lower.

From a dimensional aspect, the flat soft magnetic particles have anaverage aspect ratio (average particle diameter divided by averagethickness) of from 10 to 3,000. Further, the particles have a weightaverage particle diameter D₅₀ of 5 to 30 μm and an average thickness ofup to 1 μm.

According to another aspect of the invention, there is provided a methodfor preparing soft magnetic powder for use in magnetic shields asdefined above, comprising the steps of: furnishing particles of an alloyhaving a predetermined composition within the above-defined area incomposition diagram, flattening the alloy particles, preferably in amedia agitating mill, and heat treating the resulting flat soft magneticparticles, preferably at a temperature of from 100° to 600° C., therebycausing the particles to develop a peak corresponding to plane index(002) in an X-ray diffraction diagram.

Preferably, the soft magnetic alloy particles are heat treated prior tothe flattening step.

Also provided by the invention is a magnetic shield compositioncomprising a soft magnetic powder as defined above and a binder.

The flat soft magnetic particles which constitute the soft magneticpowder for magnetic shields according to the invention are prepared byflattening particles of an alloy having the specific composition andheat treating the resulting flat soft magnetic particles. We have foundthat alloy particles having the specific composition are prone tocleavage, particularly when they have DO₃ type crystal structure, andthus quite suitable for the manufacture of flat soft magnetic particlesintended herein.

With stresses applied, the alloy particles undergo cleavage into flatparticles. Since the cleavage planes correspond to crystal faces andhave regular directions, flattening is accompanied by a minimal loss ofmagnetic properties.

Further, because of the cleavage nature, the resulting flat softmagnetic particles have a high aspect ratio as given by average particlediameter divided by average thickness and a narrow distribution ofaspect ratio and particle diameter and are best suited for themanufacture of magnetic shields.

As to the flattening process, the time required for flattening issubstantially reduced as compared with Permalloy and other conventionalalloys which undergo flattening through rolling. This leads to efficientproduction. Use of a media agitating mill ensures quicker flatteninginto flat soft magnetic particles with consistent properties. Since thestarting alloy particles are generally prepared by rapidly quenching analloy melt or finely dividing an alloy ingot, some particles might havea distorted crystal structure. A previous heat treatment on suchparticles can tailor the crystal structure to the regular DO₃ structure,allowing the flattening process to be completed in a shorter time.

The flat soft magnetic particles of the specific composition prepared inthis way have high magnetic permeability and low coercive force,especially when they are of the DO₃ type crystal structure. They arethus best suited for the manufacture of magnetic shields.

The DO₃ type crystal structure is often lost as a result of stressesinduced during flattening. A subsequent heat treatment on flattenedparticles allows the particles to resume the DO₃ type crystal structure.

In order that non-flattened alloy particles and flattened soft magneticparticles assume the DO₃ type crystal structure, both the previous andsubsequent heat treatments may be done at temperatures as low as 100° to600° C. Therefore, the particles can be heat treated without fire riskor sintering. It is to be understood that the presence of the DO₃ typecrystal structure can be observed in an X-ray diffraction diagram as theappearance of a peak corresponding to plane index (002) characteristicof the DO₃ type crystal structure.

Further, the alloy particles of the above-defined composition can have asaturation magnetostriction constant λs of 0 or lower, avoiding any lossof magnetic permeability or any rise of coercive force by stressesapplied during flattening and during milling with a binder to form ashield composition.

The alloy of the specific composition can have negative values (lessthan zero) of saturation magnetostriction constant, it is stable againststresses in that it does not experience a loss of magnetic permeabilityor a rise of coercive force which is otherwise incurred by stressesduring flattening or during milling with binder to form a shieldingcomposition. Magnetic shields do not detract from their magneticproperties upon application of stresses during use.

In the case of Fe-Si-Cr system, a further advantage of the flat softmagnetic particles of the specific composition is high corrosionresistance. Even when they are of an extensive flat shape having anincreased specific surface area, they remain inflammable during heattreatment. They are free of any loss of magnetic properties or outerappearance due to rusting.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will be better understood from the following description takenin conjunction with the accompanying drawings, in which:

FIG. 1 is a ternary composition diagram of Fe, Si and Cr showing thealloy composition of flat soft magnetic particles as being defined andencompassed by polygon ABCDE.

FIG. 2 is an X-ray diffraction diagram of flat soft magnetic particlesof Fe-Si-Cr system which have been heat treated at 450° C. for 60minutes (sample No. 24).

FIG. 3 is an X-ray diffraction diagram of the same flat soft magneticparticles prior to heat treatment (sample No. 21).

FIG. 4 is a ternary composition diagram of Fe, Si and Al showing thealloy composition of flat soft magnetic particles as being defined andencompassed by polygon JKLMN.

FIG. 5 is an X-ray diffraction diagram of flat soft magnetic particlesof Fe-Si-Al system which have been heat treated at 500° C. for 60minutes (sample No. 225).

FIG. 6 is an X-ray diffraction diagram of the same flat soft magneticparticles prior to heat treatment (sample No. 221).

DESCRIPTION OF THE PREFERRED EMBODIMENTS Soft magnetic powder

The flat soft magnetic particles which constitute the soft magneticpowder for use in magnetic shields according to the present inventionare, in one embodiment, of an alloy having a composition defined andencompassed by pentagon ABCDE in a ternary composition diagram of Fe,Si, and Cr. The ternary composition diagram is shown FIG. 1 wherepentagon ABCDE is drawn by connecting points A, B, C, D, E, and A inthis order, provided that points A, B, C, D, and E have the followingcompositions as expressed in atomic percentage.

A: Fe₇₈ Si₂₂ Cr₀

B: Fe₇₀ Si₃₀ Cr₀

C: Fe₆₀ Si₃₀ Cr₁₀

D: Fe₆₃ Si₁₈ Cr₁₉

E: Fe₇₆ Si₁₈ Cr₆

The reason of limitation is described. Outside line BC, magnetic shieldproperties are poor. Outside line CD, the alloy has a saturationmagnetization of up to 5 kG and is unacceptable as magnetic shieldmaterial. Outside line DE, flattening requires a longer time. Outsideline EA, the alloy is less corrosion resistant and can sometimes igniteduring heat treatment.

For higher corrosion resistance, exclusion of line AB, that is,inclusion of Cr is recommended. The content of Cr is preferably at least0.1 atom%.

In one preferred embodiment, the flat soft magnetic particles are of analloy having a composition defined and encompassed by polygon FGHIE inthe ternary composition diagram of FIG. 1 wherein points F, G, H, and Ihave the following compositions as expressed in atomic percentage.

F: Fe₇₇ Si₂₀ Cr₃

G: Fe₇₁ Si₂₆ Cr₃

H: Fe₆₂ Si₂₆ Cr₁₂

I: Fe₇₀ Si₁₈ Cr₁₂

Similarly, pentagon FGHIE is drawn in the diagram of FIG. 1 byconnecting points F, G, H, I, E, and F in this order.

In another embodiment, the flat soft magnetic particles according to thepresent invention are of an alloy having a composition defined andencompassed by pentagon JKLMN in a ternary composition diagram of Fe,Si, and Al. The ternary composition diagram is shown FIG. 4 wherepentagon JKLMN is drawn by connecting points J, K, L, M, N, and J inthis order, provided that points J, K, L, M, and N have the followingcompositions as expressed in atomic percentage.

J: Fe₈₃.8 Si₉.2 Al₇

K: Fe₈₄.7 Si₉.3 Al₆

L: Fe₈₅.6 Si₁₀.4 Al₄.0

M: Fe₈₄.9 Si₁₁.1 Al₄.0

N: Fe₈₃.2 Si₉.8 Al₇

The reason of limitation is described. Outside lines JK and KL, stressapplication can incur a substantial loss of magnetic shield properties.Outside line LM, the flattening time is increased. Outside line MN,magnetic shield properties are poor. Outside line NJ, flatteningrequires a longer time.

In one preferred embodiment, the flat soft magnetic particles are of analloy having a composition defined and encompassed by polygon JKLQR inthe ternary composition diagram of FIG. 4 wherein points Q and R havethe following compositions as expressed in atomic percentage.

Q: Fe₈₅.1 Si₁₀.9 Al₄.0

R: Fe₈₃.5 Si₉.5 Al₇.0

Similarly, pentagon JKLQR is drawn in the diagram of FIG. 4 byconnecting points J, K, L, Q, R, and J in this order.

In either of the Fe-Si-Cr and Fe-Si-Al systems, the flat soft magneticparticles may contain optional elements in addition to the essentialelements. The additional elements are not particularly limited and maybe selected from metal elements, typically transition metal elements andmetalloid elements, for example, Ti, Zr, Nb, Ta, V, Mn, Mo, W, Co, Ni,Cu, Cr (for Fe-Si-Al system), Y, lanthanides, B, C and P. The content ofadditional elements is preferably 10 atom% or less, provided that thetotal of Fe, Si, and Cr or Fe, Si, and Al is 100 atom%.

The flat soft magnetic particles may contain incidental impurities suchas N, 0 and S as long as they do not adversely affect magneticproperties.

Preferably, the flat soft magnetic particles show a peak correspondingto plane index (002) in an X-ray diffraction diagram thereof. This peakindicates the presence of the DO₃ type crystal structure.

More benefits attributable to the DO₃ type crystal structure areavailable when the flat soft magnetic particles meet P(002)/P(022)≧0.1%wherein P(002) is a peak height corresponding to plane index (002) andP(022) is a peak height corresponding to plane index (022) in the X-raydiffraction diagram. It is to be noted that in an X-ray diffractiondiagram of the Fe-Si-Cr system using an Fe target, the peakcorresponding to plane index (002) appears at 2θ=39.5° and the peakcorresponding to plane index (022) appears at 2θ=57.2° In an X-raydiffraction diagram of the Fe-Si-Al system using a Cu-target, the peakcorresponding to plane index (002) appears at 2θ=31.28° and the peakcorresponding to plane index (022) appears at 2θ=44.92°.

The soft magnetic powder has a maximum magnetic permeability of 20 to80, especially 25 to 60 and a coercive force Hc of 1 to 20 Oe,especially 1 to 14 Oe.

The alloy of which the flat soft magnetic particles are formedpreferably has a negative saturation magnetostriction constant λs ofless than zero, more preferably from -10×10⁻⁶ to less than 0, mostpreferably from -3×10.6⁻⁶ to -0.01×10⁻⁶.

Now, the preferred dimensions and shape of flat soft magnetic particlesare described.

The flat soft magnetic particles have an (average) particle diameter andan (average) thickness. The average thickness should preferably be up to1 μm, more preferably 0.01 to 1 μm. Particles with an average thicknessof less than 0.01 μm are not only less dispersible in a binder inpreparing a magnetic shield composition, but are also reduced inmagnetic properties such as magnetic permeability. An average thicknessof more than 1 μm is undesirable because it is difficult to thinly coata magnetic shield composition to form a coating having flat softmagnetic particles uniformly dispersed therein. In addition, the coatinghas a less number of flat soft magnetic particles distributed in athickness direction of the coating and provides insufficient shieldingproperties. Better results are obtained with an average thickness offrom 0.01 to 0.6 μm. It is understood that the average thickness isdetermined by means of a scanning electron microscope for analysis.

The flat soft magnetic particles preferably have an average aspect ratioof from 10 to 3,000, especially from 10 to 500. By the average aspectratio used herein is meant the average diameter divided by the averagethickness of flat particles. Particles with an aspect ratio of less than10 would be greatly affected by a diamagnetic field and insufficient inmagnetic properties such as permeability and shielding properties. Flatparticles having an average thickness within the above-mentioned range,but an aspect ratio in excess of 3,000, which means that their averagediameter is too large, are susceptible to rupture during milling with abinder, resulting in a loss of magnetic properties.

The average particle diameter is a weight mean particle diameter D₅₀. Itis the diameter of flat soft magnetic particles at which the integratedvalue reaches 50% of the weight of the overall soft magnetic powder whenthe soft magnetic powder is divided into fractions of flat particles andthe weight of flat particle fractions having successively increasingdiameters is integrated from the smallest diameter fraction. Theparticle diameter is a measurement by a light scattering particlecounter. More particularly, light scattering particle size analysis iscarried out by causing particles to circulate, directing light from alight source such as a laser or halogen lamp, and measuring Fraunhoferdiffraction or the scattering angle of Mie scattering, therebydetermining the distribution of particle size. The detail of particlesize measurement is described in "Funtai To Kogyo" (Powder andIndustry), Vol. 19, No. 7 (1987). D₅₀ can be determined from theparticle size distribution obtained from the particle counter.

The flat soft magnetic particles used herein preferably have a D₅₀ of 5to 30 μm.

Desirably, the flat soft magnetic particles have a larger elongation ofat least 1.2 when the magnetic shield is required to be directional.Provided that a flat particle has a length or major diameter a and abreadth or minor diameter b along a major surface configuration, theelongation used herein is a ratio of length to breadth, a/b. If amagnetic field source to be shielded is directional, a magnetic coatingcomposition is cured while an orienting magnetic field is applied in thesame direction. Then the permeability in the direction is improved,providing an increased magnetic shield effect in the desired direction.Better results are obtained with an elongation a/b in the range of from1.2 to 5. Such an elongation is readily achievable with the use of amedia agitating mill. The length and breadth of particles can bemeasured by a transmission electron microscope for analysis.

Preparation method

It is now described how to prepare the soft magnetic powder according tothe invention. Briefly stated, the method involves furnishing alloyparticles, optionally heat treating them, flattening them, and then heattreating the flat particles.

First, particles of an alloy having a composition within pentagon ABCDEin the diagram of FIG. 1 or pentagon JKLMN in the diagram of FIG. 4 areflattened into flat soft magnetic particles. The starting alloyparticles may be prepared by conventional methods, for example, byrapidly quenching an alloy melt or finely dividing an alloy ingot.

The rapid quenching of an alloy melt is not particularly limitedalthough a water atomizing method is recommended because it can yieldalloy particles of desired size without grinding. The water atomizingmethod involves injecting water under high pressure to an alloy melt,thereby atomizing and solidifying the alloy, followed by cooling inwater. The detail of the water atomizing method is described in U.S.application Ser. No. 528,827 filed May 25, 1990 and Japanese PatentApplication No. 12267/1989.

The method for producing alloy particles is not limited to the wateratomizing method. It is also possible to produce alloy particles byinjecting a melt against a chill roll to produce ribbons, flakes orparticles. Conventional single and double chill roll methods andatomizing methods may be used. The rapidly quenched alloy may be finelydivided into alloy particles of desired size if necessary.

Where alloy particles are prepared by comminuting an alloy ingot, it isdesirable to subject the ingot to solid solution treatment prior tocomminution.

The alloy particles have an average particle diameter which may bedetermined depending on the desired diameter and aspect ratio of theflat soft magnetic particles although a weight average particle diameterD₅₀ of 5 to 30 μm, more preferably from 7 to 20 μm is preferred.

Often, the alloy particles are previously heat treated in order totailor the crystal structure, typically at a temperature of 100° to 600°C. for 10 minutes to 10 hours.

Any desired means may be employed for the purpose of flattening alloyparticles. Flattening means effective for inducing cleavage is preferredbecause alloy particle flattening proceeds mainly by way of cleavage.Such effective flattening means include a media agitating mill and atumbling ball mill, with the media agitating mill being preferred. Themedia agitating mill is a class of agitators including pin mills, beadmills, and agitator ball mills, with examples being shown in JapanesePatent Application Kokai No. 259739/1986 and U.S. Ser. No. 528,827.

A next step is to heat treat the flat particles of the desired shape anddimensions resulting from the media agitating mill. The heat treatmentcauses the material to create or resume the DO₃ type crystal structure.Typically, the flattened particles are heat treated at a temperature of100° to 600° C. for 10 minutes to 10 hours. Lower temperatures orshorter times do not achieve the purpose of heat treatment whereas thematerial can be ignited or sintered at higher temperatures. Morepreferably, the particles are heat treated at 300° to 500° C. for 30minutes to 2 hours, typically in vacuum or in an atmosphere of inert gassuch as nitrogen, hydrogen and argon. It is also acceptable to carry outheat treatment in a magnetic field.

Magnetic shield composition

The thus obtained soft magnetic powder is blended with a binder to forma magnetic shield composition in which flat particles are dispersed inthe binder.

The magnetic shield composition preferably has a maximum permeability μmof at least 50, more preferably at least 100, especially 150 to 400,most preferably 180 to 350 in a DC magnetic field and a coercive forceHc of 2 to 20 Oe, more preferably 2 to 15 Oe as calculated on theassumption that the composition consists of 100% of the powder. Thesemagnetic properties offer a satisfactory U magnetic shield effect.

The soft magnetic powder preferably occupies 60 to 95% by weight of themagnetic shield composition. If the packing is less than 60% by weight,the magnetic shield effect would be drastically reduced. If the packingis more than 95% by weight, the magnetic shield composition would bereduced in strength because the binder is too short to firmly bind softmagnetic particles together. Better magnetic shield effect and higherstrength are obtained with a packing of 70 to 90% by weight.

The binder used herein is not particularly limited. It may be selectedfrom conventional well-known binders including thermoplastic resins,thermosetting resins, and radiation curable resins.

The magnetic shield composition may contain a curing agent, dispersant,stabilizer, coupler or any other desired additives in addition to thesoft magnetic powder and the binder.

The magnetic shield composition is generally used by molding it into adesired shape, or blending it with a suitable solvent to form a coatingcomposition and applying it as a coating, and then heat curing the shapeor coating, if necessary. Curing is generally carried out in an oven ata temperature of 50° to 80° C. for about 6 to about 100 hours althoughcuring conditions depend on a particular type of binder.

When it is desired to shape the magnetic shield composition into a filmor thin band which is suitable as a magnetic shield, the film or thinband preferably has a thickness of 5 to 200 μm. Since the magneticshield composition of the invention has magnetic properties aspreviously defined, films as thin as 5 μm can have a magnetic shieldingeffect. For shielding against a magnetic field having an intensity atwhich the shield composition is not magnetically saturated, the magneticshielding effect is increased no longer by increasing the thickness of afilm beyond 200 μm. The maximum thickness of 200 μm is also determinedfor economy.

When the magnetic shield composition is molded into a desired shape orcoated, a directional magnetic shield can be produced by applying anorienting magnetic field or effecting mechanical orientation.Particularly when the magnetic shield composition is formed into a plateor film having a thickness within the above-defined range, the plate orfilm shows a high magnetic shielding effect against a magnetic fieldparallel to the major surface thereof.

When used in the magnetic shield composition, the soft magnetic powdermay be formed with a conductive coating of Cu, Ni or a similar metal.

EXAMPLE

Examples of the present invention are given below by way of illustrationand not by way of limitation.

Examples 1-3 relate to the Fe-Si-Cr system.

EXAMPLE 1

Flat soft magnetic particles of different compositions were prepared toshow the effectiveness of the invention.

Alloy particles were prepared by the water atomizing method, flattenedby means of a media agitating mill, and then heat treated, obtaining asoft magnetic powder consisting of flat soft magnetic particles.

Table 1 shows the composition of flat soft magnetic particles and theholding temperature and time during the heat treatment.

Flattening in the medium agitating mill was conducted until the weightaverage particle diameter D₅₀ of flat soft magnetic particles reached 15μm. The time taken for flattening was measured. The results are shown inTable 1.

The average thickness was measured by a scanning electron microscope foranalysis and D₅₀ measured by means of a light scattering particlecounter.

After the heat treatment, the flat soft magnetic particles were subjectto X-ray diffraction analysis using an Fe target. From the X-raydiffraction diagram, the peak heights P(002) and P(022) at plane indexes(002) and (022) were determined to calculate P(002)/P(022).

The results of X-ray diffraction analysis are also shown in Table 1.

The alloy of each composition was measured for saturationmagnetostriction constant λs, with the results shown in Table 1.

To examine corrosion resistance, these soft magnetic powders were dippedin 5% NaCl at 20° C. for 48 hours. Corrosion resistance was evaluatedaccording to the following criterion.

∘: outer appearance unchanged

Δ: slight color change

X: rust over the entire surface

A magnetic shield composition was prepared by mixing each soft magneticpowder with the following binder, curing agent and solvent.

    ______________________________________                                                             Parts by weight                                          ______________________________________                                        Binder                                                                        Vinyl chloride-vinyl acetate copolymer                                                               100                                                    (Eslek A, Sekisui Chemical K.K.)                                              Polyurethane (Nippolan 2304, Nihon                                                                   100                                                    Polyurethane K.K.), calculated as solids                                      Curing agent                                                                  Polyisocyanate (Colonate HL, Nihon                                                                    10                                                    Polyurethane K.K.)                                                            Solvent                                                                       Methyl ethyl ketone    850                                                    ______________________________________                                    

The magnetic shield composition contained 80% by weight of the softmagnetic powder.

The magnetic shield composition was applied to a length of polyethyleneterephthalate film of 75 μm thick to form a coating of 25 μm thick. Thecoated film was taken up in a roll form, which was heated at 60° C. for60 minutes to cure the binder. The coated film was cut into sectionswhich were used as shield plates. Table 1 reports a coercive force (Hc)calculated on a 100% powder basis as one representative magneticproperty of the shield plate.

The shield plate was measured for shielding ratio as follows. Theshielding plate was placed on a magnet to determine a leakage magneticflux φ at a position spaced 0.5 cm from the plate. The shielding ratio(φ/φ0) was determined by dividing the leakage magnetic flux φ by themagnetic flux φ0 determined without the shielding plate. The shieldingratio is calculated based on a shielding ratio of 100 for sample No. 1.Samples having a shielding value of 150 or lower are acceptable formagnetic shielding although lower shielding ratios are preferred.

For comparison purposes, soft magnetic powders were prepared byflattening particles of Sendust alloy, Permalloy, Molybdenum Permalloyand Fe base amorphous alloy under the same conditions as in Example 1.They were examined and evaluated by the same tests as above. The resultsare also shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                    Heat             Flatten-                                     Sample          Treatment                                                                           P(002)/P(022)                                                                        λs                                                                         ing Time                                                                           Corrosion                                                                           Hc Shield                         No. Composition (°C./min.)                                                                   (%)    (10.sup.-6)                                                                       (hour)                                                                             resistance                                                                          (Oe)                                                                             ratio**                                                                           Remarks                    __________________________________________________________________________    1   Si.sub.20 Cr.sub.8 bal.Fe (at %)                                                          450 × 60                                                                      3.5    -5  5    ◯                                                                        7 100                            2   Si.sub.20 Cr.sub.5 bal.Fe (at %)                                                          450 × 60                                                                      3.2    -6  5    ◯                                                                        8 105                            3   Si.sub.25 Cr.sub.5 bal.Fe (at %)                                                          450 × 60                                                                      3.6    -8  4    ◯                                                                        8 100                            4   Si.sub.30 Cr.sub.5 bal.Fe (at %)                                                          450 × 60                                                                      3.0    -4  5    ◯                                                                       10 110                            5   Si.sub.25 Cr.sub.5 Al.sub.3 bal.Fe (at %)                                                 450 × 60                                                                      3.4    -5  4    ◯                                                                       10 110                            6   Si.sub.25 Cr.sub.5 Nb.sub.3 bal.Fe (at %)                                                 450 × 60                                                                      2.8    -2  4    ◯                                                                        9  95                            7   Si.sub.25 Cr.sub.5 Mn.sub.3 bal.Fe (at %)                                                 450 × 60                                                                      2.8    -4  4    ◯                                                                       10 110                             8* Si.sub.35 Cr.sub.5 bal.Fe (at %)                                                          450 × 60                                                                      0.1    -15 7    ◯                                                                       19 250                             9* Si.sub.25 Cr.sub.20 bal.Fe (at %)                                                         450 × 60                                                                      2.0    -10 4    ◯                                                                       15 210                            10* Si.sub.15 Cr.sub.10 bal.Fe (at %)                                                         450 × 60                                                                      0.5    +3  8    ◯                                                                       20 300                            11* Si.sub.9.6 Al.sub.5.4 bal.Fe (wt %)                                                       NO    (002) NONE                                                                           ˜0                                                                          5    X     15 140 Sendust                    12* Ni.sub.50 bal.Fe (wt. %)                                                                  NO    (002) NONE                                                                           +20 16   X     20 250 Permalloy                  13* Ni.sub.80 Mo.sub.5 bal.Fe (wt %)                                                          NO    (002) NONE                                                                           ˜0                                                                          16   Δ                                                                             15 180 Molybdenum Permalloy       14* Cr.sub.4 Nb.sub.3 Si.sub.18 B.sub.6 bal.Fe                                                450 × 60                                                                      (002) NONE                                                                           +15 12   ◯                                                                        4 120 Amorphous alloy                (at %)                                                                    __________________________________________________________________________     *comparison                                                                   **relative value based on No. 3 = 100                                    

The effectiveness of the invention is evident from Table 1.

More particularly, sample Nos. 1-7 within the scope of the inventionshow a shorter flattening time, high corrosion resistance, and anegative value of saturation magnetostriction constant λs. They alsosatisfy the low coercive force requirement as magnetic shields andprovide high shielding ratios when formed into magnetic shield plates.

Sample Nos. 8-10 outside the scope of the invention show poor magneticshield properties and sample No. 10 requires a longer time to flatten.

Sample No. 11 or Sendust alloy is less corrosion resistant. Sample Nos.12-14 require at least twice longer time to flatten than the presentsamples and are low in productivity. Sample No. 12 or Permalloy showslow corrosion resistance, high magnetostriction, and poor shieldingproperties. Sample No. 13 or Molybdenum Permalloy is unacceptable incorrosion resistance and shielding properties. Sample No. 14 or Fe baseamorphous alloy has high magnetostriction so that the desired shieldingeffect is lost when stresses are applied to shield members.

EXAMPLE 2

The properties of sample No. 3 in Table 1 were examined while the heattreating conditions were varied. The test conditions are the same as inExample 1.

The results are shown in Table 2.

Sample Nos. 24 and 21 were analyzed by X-ray diffraction using an Fetarget. X-ray diffraction diagrams are shown in FIGS. 2 and 3.

                  TABLE 2                                                         ______________________________________                                              Heat                                                                    Sample                                                                              treatment P(002)/P(022)                                                                            Hc   Shielding                                     No.   °C./min.                                                                         %          Oe   ratio  Remarks                                ______________________________________                                        21    no        (002) none 22   350                                           22    250/60    1.5        13   180                                           23    350/60    1.8        12   130                                           24    450/60    3.6         8   100    = No. 3                                25    550/60    4.1         7   100                                           26    650/60    unmeasurable                                                                             50   700    burnt                                  ______________________________________                                    

EXAMPLE 3

Alloy particles of the compositions shown in Table 1 were heat treatedat 450° C. for one hour before flattening under the same conditions asin Example 1. The time taken for flattening was reduced by 10% or more.

Examples 4-6 relate to the Fe-Si-Al system.

EXAMPLE 4

Flat soft magnetic particles of different compositions were prepared toshow the effectiveness of the invention.

Alloy particles were prepared by the water atomizing method, flattenedby means of a media agitating mill, and then heat treated, obtaining asoft magnetic powder consisting of flat soft magnetic particles.

Table 3 shows the composition of flat soft magnetic particles and theholding temperature and time during the heat treatment.

Flattening in the medium agitating mill was conducted until the weightaverage particle diameter D₅₀ of flat soft magnetic particles reached 15μm. The time taken for flattening was measured. The results are shown inTable 3.

The average thickness was measured by a scanning electron microscope foranalysis and D₅₀ measured by means of a light scattering particlecounter.

After the heat treatment, the flat soft magnetic particles were subjectto X-ray diffraction analysis using a Cu target. From the X-raydiffraction diagram, the peak heights P(002) and P(022) at plane indexes(002) and (022) were determined to calculate P(002)/P(022).

The results of X-ray diffraction analysis are also shown in Table 3.

The alloy of each composition was measured for saturationmagnetostriction constant λs. An alloy sample of 5×5×20 mm was heattreated as in Table 3 and measured by the three terminal capacitymethod, with the results shown in Table 3.

A magnetic shield composition was prepared by mixing each soft magneticpowder with the following binder, curing agent and solvent.

    ______________________________________                                                             Parts by weight                                          ______________________________________                                        Binder                                                                        Vinyl chloride-vinyl acetate copolymer                                                               100                                                    (Eslek A, Sekisui Chemical K.K.)                                              Polyurethane (Nippolan 2304, Nihon                                                                   100                                                    Polyurethane K.K.), calculated as solids                                      Curing agent                                                                  Polyisocyanate (Colonate HL, Nihon                                                                   10                                                     Polyurethane K.K.)                                                            Solvent                                                                       Methyl ethyl ketone    850                                                    ______________________________________                                    

The magnetic shield composition contained 80% by weight of the softmagnetic powder.

The magnetic shield composition was applied to a length of PET film of75 μm thick to form a coating of 25 μm thick. The coated film was takenup in a roll form, which was heated at 60° C. for 60 minutes to cure thebinder. The coated film was cut into sections which were used as shieldplates. Table 1 reports a coercive force (Hc) calculated on a 100%powder basis as one representative magnetic property of the shieldplate.

The shield plate was measured for shielding ratio as follows. Theshielding plate was placed on a magnet to determine a leakage magneticflux φ at a position spaced 0.5 cm from the plate. The shielding ratio(φ/φ0) was determined by dividing the leakage magnetic flux φ by themagnetic flux φ0 determined without the shielding plate. The shieldingratio is calculated based on a shielding ratio of 100 for sample No.201. It is to be understood that samples having a shielding value of 150or lower are acceptable for magnetic shielding purposes although lowershielding ratios are preferred.

To examine the influence of stress application on magnetic shieldproperties, a certain load was applied to a magnetic shield sample tomeasure a change in magnetic shield properties. The results are shown inTable 3.

For comparison purposes, soft magnetic powders were prepared byflattening particles of Sendust alloy, Permalloy, Molybdenum Permalloyand Fe base amorphous alloy under the same conditions as in Example 3.They were examined and evaluated by the same tests as above. The resultsare also shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________                    Heat              Flattening   Change                         Sample                                                                            Composition Treatment                                                                            P(002)/P(022)                                                                        λs                                                                         Time  Hc Shield                                                                            under stress                   No. Fe Si Al    (°C./min.)                                                                    (%)    (10.sup.-6)                                                                       (hour)                                                                              (Oe)                                                                             ratio**                                                                           (%)    Remarks                 __________________________________________________________________________    201 84.0                                                                              9.5                                                                             6.5 (wt %)                                                                          450 × 60                                                                       3.5    -1.0                                                                              4.5   5.0                                                                              100 -2                             202 84.7                                                                              9.8                                                                             5.5 (wt %)                                                                          450 × 60                                                                       4.5    -0.7                                                                              4.5   4.0                                                                              100  0                             203 85.3                                                                             10.2                                                                             4.5 (wt %)                                                                          450 × 60                                                                       3.5    -0.3                                                                              4.8   5.0                                                                              107 -5                             204*                                                                              82.8                                                                             10.0                                                                             7.2 (wt %)                                                                          450 × 60                                                                       2.0    -0.4                                                                              8.0   8.0                                                                              165 -10                            205*                                                                              86.4                                                                             10.2                                                                             3.4 (wt %)                                                                          450 × 60                                                                       1.5    1.0 7.0   9.0                                                                              170 80                             206*                                                                              85.0                                                                             11.6                                                                             3.4 (wt %)                                                                          450 × 60                                                                       2.0    -3.0                                                                              7.0   8.0                                                                              165 -8                             207*                                                                              83.8                                                                              9.0                                                                             7.2 (wt %)                                                                          450 × 60                                                                       1.8    0.5 7.0   10.0                                                                             190 60                             208*                                                                              85.0                                                                              9.6                                                                             5.4 (wt %)                                                                          NO     NONE   0.1 5.0   15.0                                                                             185 20     Sendust alloy           209*                                                                              85.0                                                                              9.6                                                                             5.4 (wt %)                                                                          450 × 60                                                                       5.6    0.1 5.0   12.0                                                                             150 30     Sendust alloy           210*                                                                              Fe.sub.50 Ni.sub.50 (wt %)                                                                NO     NONE   20.0                                                                              16.0  20.0                                                                             250 350    Permalloy               211*                                                                              Ni.sub.80 Mo.sub.5 Fe.sub.15 (wt %)                                                       NO     NONE   0.0 16.0  15.0                                                                             180 35     Permalloy               212*                                                                              Fe.sub.69 Cr.sub.4 Nb.sub.3 Si.sub.18 B.sub.6                                             450 × 60                                                                       NONE   15.0                                                                              12.0  4.0                                                                              120 230    Amorphous alloy             (at %)                                                                    __________________________________________________________________________     *comparison                                                                   **relative value based on No. 201 = 100                                  

The effectiveness of the invention is evident from Table 3.

More particularly, sample Nos. 201-203 within the scope of the inventionshow a shorter flattening time and a negative value of saturationmagnetostriction constant λs. They also satisfy the low coercive forcerequirement as magnetic shields, provide high shielding ratios whenformed into magnetic shield plates, and maintain such shielding propertyunchanged upon stress application.

Sample Nos. 204-207 having a composition outside line JN or LM in FIG. 4require a longer time to flatten and show poor magnetic shieldproperties.

Sample Nos. 205 and 207 having a composition outside line JK or KL inFIG. 4 and sample No. 209 (Sendust alloy) experience a substantial lossof magnetic shield properties upon stress application.

Sample Nos. 204 and 206 having a composition outside line MN is ratherincreased in magnetic shield properties upon stress application, butgenerally poor in all the aspects.

Sample Nos. 210 to 212 corresponding to Permalloy and Fe base amorphousalloy are poor in productivity since they require at least two or threetimes longer time to flatten than the present samples. Their shieldingproperties are poor and deteriorated upon stress application.

EXAMPLE 5

An alloy having the composition 85.1 wt % Fe-10.1 wt % Si-4.8 wt % Alwithin the scope of the invention was measured for various propertieswhile the heat treating conditions were varied. The test conditions arethe same as in Example 4.

The results are shown in Table 4.

Sample Nos. 225 and 221 were analyzed by X-ray diffraction using a Cutarget. X-ray diffraction diagrams are shown in FIGS. 5 and 6.

                  TABLE 4                                                         ______________________________________                                              Heat                                                                    Sample                                                                              treatment P(002)/P(022)                                                                            Hc   Shielding                                     No.   °C./min.                                                                         %          Oe   ratio  Remarks                                ______________________________________                                        221   no        (002) none 17   200                                           222   200/60    1.0        4    105                                           223   300/60    1.7        5    101                                           224   400/60    3.6        8     95                                           225   500/60    4.6        9    100                                           226   700/60    unmeasurable                                                                             --   --     ignited                                ______________________________________                                    

EXAMPLE 6

Alloy particles of the compositions shown in Table 3 were heat treatedat 450° C. for one hour before flattening under the same conditions asin Example 4. The time taken for flattening was reduced by 10% or more.

The effectiveness of the invention is evident from the examples.

The flat soft magnetic particles of which the soft magnetic powder ofthe invention is comprised are quite suitable for producing magneticshields since they have high magnetic permeability, a low coerciveforce, a saturation magnetostriction constant λs which can be 0 ornegative, and high corrosion resistance.

Since the starting material is alloy particles susceptible to cleavage,flat soft magnetic particles having a high aspect ratio can be producedbriefly. Since flattening is followed by heat treatment to create thedesired crystal structure, there are obtained particles havingsatisfactory magnetic properties.

The magnetic shield composition using such soft magnetic powder isinexpensive, effective in performance and thus applicable as magneticshields for use in various electrical equipment such as speakers andcathode ray tubes (CRT).

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in the light of theabove teachings. It is therefore to be understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

We claim:
 1. A soft magnetic powder for use in magnetic shieldcomprising flat soft magnetic particles of an alloy having a compositiondefined and encompassed by polygon ABCDE in a ternary compositiondiagram of Fe, Si, and Cr wherein points A, B, C, D, and E have thefollowing compositions as expressed in atomic percentageA: Fe₇₈ Si₂₂ Cr₀B: Fe₇₀ Si₃₀ Cr₀ C: Fe₆₀ Si₃₀ Cr₁₀ D: Fe₆₃ Si₁₈ Cr₁₉ E: Fe₇₆ Si₁₈ Cr₆wherein said flat soft magnetic particles have a weight average particlediameter D₅₀ of 5 to 30 μm and an average thickness of up to 1 μm, theaverage particle diameter divided by the average thickness being from 10to 3,000.
 2. The soft magnetic powder of claim 1 wherein the flat softmagnetic particles show a peak corresponding to plane index (002) in anX-ray diffraction diagram thereof.
 3. The soft magnetic powder of claim2 wherein the flat soft magnetic particles meet P(002)/P(022)≧0.1%wherein P(002) is a peak height corresponding to plane index (002) andP(022) is a peak height corresponding to plane index (022) in the X-raydiffraction diagram.
 4. The soft magnetic powder of claim 2 wherein saidalloy has a negative saturation magnetostriction constant λs.
 5. Amagnetic shield composition comprising a soft magnetic powder as setforth in any one of claim 1 to 4 and a binder.
 6. A soft magnetic powderfor use in magnetic shields comprising flat soft magnetic particles ofan ally having a composition defined and encompassed by polygon JKLMN ina ternary composition diagram of Fe, Si, and Al wherein points J, K, L,M, and N have the following compositions as expressed in atomicpercentage:J: Fe₈₃.8 Si₉.2 Al₇ K: Fe₈₄.7 Si₉.3 Al₆ L: Fe₈₅.6 Si₁₀.4Al₄.0 M: Fe₈₄.9 Si₁₁.1 Al₄.0 N: Fe₈₃.2 Si₉.8 Al₇ wherein the flat softmagnetic particles show a peak corresponding to plane index (002) in anX-ray diffraction diagram thereof and wherein said flat soft magneticparticles have a weight average particle diameter D₅₀ of 5 to 30 μm andan average thickness of up to 1 μm.
 7. The soft magnetic powder of claim6 wherein the flat soft magnetic particles meet P(002)/P(022)≧0.1%wherein P(002) is a peak height corresponding to plane index (002) andP(022) is a peak height corresponding to plane index (022) in the X-raydiffraction diagram.
 8. The soft magnetic powder of claim 6 wherein saidalloy has a saturation magnetostriction constant λs of at most zero. 9.The soft magnetic powder of claim 6 wherein said flat soft magneticparticles have an average particle diameter and an average thickness,the average particle diameter divided by the average thickness beingfrom 10 to 3,000.
 10. A magnetic shield composition comprising a softmagnetic powder as set forth in any one of claim 6 to 9 and a binder.11. A soft magnetic powder for use in magnetic shields comprising flatsoft magnetic particles of an alloy having a composition defined andencompassed by polygon FGHIE in a ternary composition diagram of Fe, Si,and Cr wherein points F, G, H, I, and E have the following compositionas expressed in atomic percentageF: Fe₇₇ Si₂₀ Cr₃ G: Fe₇₁ Si₂₆ Cr₃ H:Fe₆₂ Si₂₆ Cr₁₂ I: Fe₇₀ Si₁₈ Cr₁₂ E: Fe₇₆ Si₁₈ Cr₆ wherein said flat softmagnetic particles have a weight average particle diameter D₅₀ of 5 to30 μm and an average thickness of up to 1 μm, the average particlediameter divided by the average thickness being from 10 to 3,000.
 12. Asoft magnetic powder for use in magnetic shields comprising flat softmagnetic particles of an alloy having a composition defined andencompassed by polygon JKLQR in a ternary composition diagram of Fe, Si,and Al wherein points j, K, L, Q, and R have the following compositionsas expressed in atomic percentage:J: Fe₈₃.8 Si₉.2 Al₇ K: Fe₈₄.7 Si₉.3Al₆ L: Fe₈₅.6 Si₁₀.4 Al₄.0 Q: Fe₈₅.1 Si₁₀.9 Al₄.0 R: Fe₈₃.5 Si₉.5Al₇.0wherein the flat soft magnetic particles show a peak correspondingto plane index (002) in an X-ray diffraction diagram thereof and whereinsaid flat soft magnetic particles have a weight average particlediameter D₅₀ of 5 to 30 μm and an average thickness of up to 1 μm.